This invention relates to the generation of microbubbles of gas in a liquid for all applications where such microbubbles are desired and to generating and dispensing of microbubbles, particularly at the wetted surfaces of navigable vessels whereby to provide the vessel with improved performance by decreasing frictional resistance and turbulence. A vessel moving through water experiences frictional resistance at the wetted surface below the water line. As the speed of the vessel increases the turbulence created by the hull moving through the water increases rapidly until frictional forces become the practical barrier to higher speed. The energy required to propel the vessel increases correspondingly. Improving speed and efficiency are recognized as the primary goals and activities in the naval arts and decreasing frictional resistance is seen as the key to these goals.
Conventional devices for bubble generation as, for example those described in European Patent Application No. 84110151.2 (publication no. 0 135 822), constitute gas permeable plates with means for introducing gas under pressure on one side so that it is forced through the plate to the other side and into a liquid which is at the other side. Usually such plates are made porous by mechanical treatment, as by punching, piercing, boring, etc. such that the bubbles produced are relatively large and/or the throughput at reasonable pressures is quite limited. Smaller bubbles on the order of 50 microns or less (i.e. microbubbles) have been produced utilizing microporous metal plates made of powdered metal which is then sintered and subjected to an etching process to create the pores, as described in the papers of Madavan, Deutsh and Merkle published in the Journal of Fluid Mechanics (1985), vol. 156, pp. 237-245, titled "Measurement of local skin friction in a microbubble-modified turbulent boundary layer" and in Phys. Fluids 27 (2), February 1984, entitled "Reduction of turbulent friction by microbubbles" and in the references cited in these papers. However the energy required for producing the microbubbles utilizing these plates is so great as to even exceed the energy saved in the application of the microbubbles thus generated for reduction of skin friction at the interface between the bubble-laden water and the wetted surface along which it moved.
In International Patent Application No. PCT/SE86/00114 fabrics with micropores, particularly polyester fabrics, are suggested for producing microbubbles. A flexible membrane forming a wall of the plenum behind the microporous fabric is subjected to mechanical vibrations to create pressure variations, presumably to promote bubble production. It is unlikely that such fabrics with or without vibrations of the magnitude contemplated would be capable of producing microbubbles in adequate volume and at a reasonable pressure differential. It is even less likely that the microbubbles would be of relatively uniform size and without a substantial fraction of larger bubbles that are not only inefficient but also detrimental to some applications such as friction reduction at the wetted surface of a vessel. As microbubbles having average diameters of less than 60 microns have been found to be the most effective in reducing friction and for other uses as well, if they can be produced in adequate volume economically and without the coproduction of larger bubbles, the foregoing prior art presents serious limitations.
In a marine environment, bubble generating devices are constantly exposed to the attack of microorganisms, minute aquatic life and other foreign matter that can quickly foul a microporous material used for bubble generation. Introduction of anti-fouling chemicals has been suggested but they are difficult to apply on an ongoing basis and are only of limited effectiveness. Ultrasonic energy has been employed for cleaning materials in a liquid bath by so-called cavitation cleaning by the implosion of vapor bubbles generated in the liquid by the wave energy as illustrate in U.S. Pat. Nos. 4,525,219, 4,253,962 and 4,346,011. In the latter two patents, the wave energy scours off particles at the influent side of a screen or microporous filter that impede transfer of the liquid therethrough, by causing cavitation in the liquid to remove or agglomorate the particles. In addition to other conventional techniques ultrasonic cleaning, presumably in a liquid bath by the cavitation technique, has been suggested for cleaning fiber metal acoustic materials. However, ultrasonic energy has not heretofore been applied in the generation of microbubbles.
Fiber metal is the descriptive name that has been given to felted metal fibers which are produced in various configurations including molded shapes, sheets, rings, pads, etc. and in varying degrees of porosity and density. These materials have been employed in accoustical applications, abrading pads and as a filter, particularly for gases. A fiber metal sheet construction has also been used for as a diffuser for Argon to provide an inert atmosphere adjacent the outflow face of the sheet for welding operations. Heretofore, fibermetal has not been employed in connection with the generation of microbubbles.
An extremely thin layer of water immediately next to the surface of the vessel below the water line(wetted surface) is termed the boundary layer. In this layer most of the shear forces of frictional resistance take place. Microscopic air or other gas bubbles, i.e. microbubbles, have been proposed for reducing friction on a vessel by their introduction into this boundary layer. This technique appears to have potential but there are serious drawbacks and limitations in the means which have been proposed for generating microbubbles, and the means proposed for introducing and distributing them.
PCT/SE86/00114 and other references disclose positioning of a microporous material flush with the hull wall of a vessel in order to distribute bubbles into the flow adjacent the hull. As friction reduction is effected only adjacent and aftwardly of the device, such placement correspondingly limits their effectiveness, particularly at the bow regions where friction reduction can produce the most effect.
Various proposals have been made for dispensing bubbles in advance of a vessel, as for example shown in U.S. Pat. Nos. 420,670, 661,303, 1,822,223, 2,378,822, 3,534,699 and 3,972,494. However, in each case the shape and configuration of the dispensing apparatus is such as would create turbulence and drag that would at the least severely limit overall effectiveness and/or the location of the apparatus with respect to the hull would greatly limit or negate effectiveness.
Each of U.S. Pat. Nos. 661,303 and 2,378,822 disclose an attachment for the bow of a vessel which is mounted over the apex and extends aft for a distance. Gas is discharged at holes or ports along the aft margins. At least in the case of U.S. Pat. No. 2,378,822 the forwardly facing portion of the attachment is streamlined to match to some extent the bow which it covers. This will reduce to some degree turbulence created by the attachment as compared to a totally bluff body (having blunt shapes that create a rapid increase of pressure gradient downstream). However, the added resistance and drag due to this structure will be substantial because elements of the attachment necessarily extend to the outside of the bow, thus creating turbulence, particularly at the necked in stern portion and at the discontinuities created by the sternward edges. Additionally as the location of the bubble release is aft of the bow apex, there is no coverage at the area which is typically the most important with regard to creation of turbulence. Nor are these attachments capable of effectively spreading bubbles appreciably outward of the boundary layer at the bow for the purpose of reducing viscosity in the sublayers.
In the other disclosures nozzles or perforated diffuser pipes are positioned below the waterline in advance of the bow to emit bubbles, using air supplied by connecting pipes from the vessel. Full coverage with bubbles of the field in advance of the bow is possible in this fashion. However, for the most part these systems are constituted of bluff bodies which themselves cause appreciable turbulence in the water passing the bow, thus they detract significantly from any friction reduction on the hull by the emitted bubbles. In U.S. Pat. Nos. 1,822,223 and 3,534,699 some streamlining is provided for the bubble dispensing devices, in the former patent perforated flat plates and oval pipes positioned directly in front of the side and bottom margins of the hullat its widest and a tapered bulb-shaped aspirator confronting the bow at the keel line in the later. However, such shapes are far from optimal in minimizing frictional resistance and turbulence and the design and placement of these devices with respect to the bow severely limit the effective distribution of the microbubbles adjacent the oncoming vessel. The bow dispaces water outwardly and therefore dispensing bubbles from points confronting the frontal, full-width profile of the hull will result in movement of many of the bubbles too far outboard of the hull as it passes. Dispensing from a concentrated location as with the bulb aspirator also greatly limits effective distribution of the bubbles adjacent the bow.